1. Field of the Invention
This invention relates to a method for the manufacture of phthalic anhydride of high purity by refining crude phthalic anhydride obtained by vapor-phase catalytic oxidation of ortho-xylene. To be more particular, this invention relates to a method for the manufacture of high-purity phthalic anhydride by effecting efficient removal of such impurities as phthalide which are contained in the aforementioned crude phthalic anhydride and are difficult of separation therefrom.
2. Description of Prior Arts
Generally it is widely practiced to obtain phthalic anhydride by subjecting ortho-xylene, the raw material, to vapor-phase catalytic oxidation by use of a vanadium type catalyst. Into the crude phthalic anhydride obtained by this method contains impurities represented by by-produced phthalide. Such impurities cannot be sufficiently separated by distillation devices of the class ordinarily used commercially. Thus, refined phthalic anhydride has phthalide and other impurities persisting therein in hardly negligible amounts. It is well known that owing to these persistent impurities, the produced phthalic anhydride is liable to suffer from degradation of quality.
The process for the production of phthalic anhydride, therefore, requires the vapor-phase catalytic oxidation of ortho-xylene to be controlled so that the amount of phthalide suffered to occur as an impurity in the crude phthalic anhydride may be decreased to the fullest extent and retained at a low level. Generally it is inevitable, therefore, to elevate the reaction temperature for the purpose of curbing the decrease in the conversion of ortho-xylene and the increase in the amount of intermediate by-product phthalide due to a decline in the activity of the catalyst. This elevation of the reaction temperature results in a decrease in the service life of the catalyst. Even apart from this difficulty, the unwanted presence of by-produced phthalide in the crude phthalic anhydride is inevitable. In the commercial production of phthalic anhydride, various methods have been suggested for removal of the phthalide.
Japanese Patent Publication No. 10333/1970, for example, discloses a method which comprises treating the crude phthalic anhydride with a sulfur compound of alkali metal such as potassium hydrogen sulfite (KHSO.sub.3) or potassium pyrosulfate (K.sub.2 S.sub.2 O.sub.5) and U.S. Pat. No. 4,165,324 discloses a method which comprises effecting this treatment with an alkali metal hydroxide such as sodium hydroxide (NaOH) or potassium hydroxide (KOH). These methods, however, have been blamed for the following disadvantages, which are held to make their commercially beneficial adoption infeasible.
First, the former method has a disadvantage that since the residue originating in the refining step of phthalic anhydride contains a sulfur compound, a large amount of sulfur oxide occurs during the disposal (as by incineration, for example) of the residue and the sulur oxide induces the problem of air pollution, that the sulfur compound itself is liable to cause corrosion of the distillation device, and that a huge expense is required for the solution of these problems. Then, in the case of the latter method, since the reactivity of the alkali metal hydroxide itself is high, the treatment can become very dangerous depending on the particular type of crude phthalic anhydride to which the alkali metal hydroxide is to be added. When potassium hydroxide is added to crude phthalic anhydride in a liquid state, for example, maleic anhydride contained therein is explosively polymerized to form a polymer of the form of coke. This reaction is so violent as to have a hardly negligible effect upon the operation of the system used for the production of phthalic anhydride. Moreover, the resultant polymer of the form of coke can cause clogging in pipes, valves, and trays in the distillation tower. Further, the violent reaction of potassium hydroxide with maleic anhydride induces decomposition of phthalic anhydride and the impurities, with the result that the decomposition product reacts with phthalic anhydride to produce a polymer of the form of tar. The yield of refined phthalic anhydride, consequently, is heavily lowered.
Methods directed to obtaining phthalic anhydride of high purity by oxidizing phthalide with an oxidizing catalyst have also been proposed. U.S. Pat. No. 3,201,423, for example, discloses a method which comprises adding a heavy metal bromide such as cobalt bromide or manganese bromide to the crude phthalic anhydride and exposing the resultant mixture to contact with molecular oxygen gas. This method, however, is commercially infeasible because the bromide used in the catalyst is expensive and the recovery of the used catalyst for reuse is extremely difficult. For a similar purpose, West German Patent Disclosure No. 1,935,008 teaches a method which comprises forming a packed bed of catalyst having vanadium oxide deposited on a support and passing the crude phthalic anhydride through the packed bed while delivering air thereto thereby effecting oxidation of phthalide. It has been found, however, that when this method is adopted, the tarry substance present in the crude phthalic anhydride adheres to the surface of the supported catalyst and the catalyst is totally deprived of its activity in a very short time.
Generally, a support catalyst is formed by efficiently dispersing and depositing a catalytically active substance on the surface of a support. It is well known that even in accordance with this method of manufacture, the catalytic activity of the produced catalyst is largely affected such as by the burning condition. It is, therefore, only logical to conclude that any method which attempts to utilize such a supported catalyst placed in a packed column as a fixed bed will not be readily carried out stably on a commercial scale.
An object of this invention, therefore, is to provide a novel method for the manufacture of phthalic anhydride of high purity.
Another object of this invention is to provide a method for producing phthalic anhydride of high purity by effecting oxidation of phthalide by use of a catalyst which permits very easily controlled handling.